Nanofabrication Techniques Dominique Mailly Laboratoire de Photonique et de Nanostructures Marcoussis
Summary Introduction Optical Lithography X-ray lithography E-beam Lithography Ion beam Lithography Near field Lithography Soft and Imprint Lithography Transfert techniques
Typical Flowchart for fabrication substrat Resist spinning exposure development Metal deposition lift-off etching Electrolytic growth
lithography Crucial step which will fix the size of the pattern Lentille g contact 1:1 projection 1:5 à 1:20 Focused beam writing
Moore Law
Resist and contrast Contrairely to photography one does not want any gray scale The highest contrast is the best. negative resist positive resist
Optical lithography by contact or proximity gap b b b e Light intensity on resist mask resist wafer Ideal transfert Real transfert resolution limited by diffraction: t= λg gap minimum=resist thickness Substrats flatness Resist damage Mask damage mask1:1 e.g. g=10µm, l=400nm t=10µm Typically in a lab one can achieve 0.5µm and reach 0.2µm with conformal masks λ >200nm for mask transparency Simple and economical this is the popular lithographic tool for labs and R&D for intermediate resolution
Projection Lithography UV light Resolution limited by diffraction: R = k λ N.A. N.A. numerical aperture k technological parameter theoretical k=0.61 (Rayleigh criteria) process parameter lens aperture DOF N.A. -2 NA=n sini i 1:5 to 1:20
Evolution of projection lithography Year λ N.A. resolution k 1980 436nm 0.28 1.25µm 0.8 1990 365nm 0.48 0.5µm 0.65 1995 248nm 0.5 0.3µm O.6 1999 248nm 0.63 0.18µm 0.46 2003 193nm 0.6 0.090µm 0.49 k<k Rayleigh top imaging technique and phase shift mask
Top imaging technique and phase shift mask k : 0.61 0.4
10M! 193nm lithography
Refractive mask Reflection mask
EUV Lithography EUV are absorbed by all material and gases: need to be in vacuum
At the moment the situation is not clear between 157nm/immersion lens and EUV
X-ray lithography X photon Choice of wave length: diffraction t=(λg) 1/2 mean free path of photo-electron:l λ α mask transparency absorber efficiency Photo-electron extension 0.8nm < λ < 1.6nm Not sensitive to dust particles large process lattitude diverging source enlargment and shadow Parallel source synchrotron light
X-ray mask no X ray optics mask 1:1 absorber:au, W, Ta 0.4µm membrane: Si 3 N 4, SiC 2µm Stand Si Need to control stress of membrane for flatness No stress in absorber Good mechanical stability The major difficulty of X-ray lithography
Example of X-ray lithography 30 nm lines onpmma 20 nm dots on PMMA X-ray lithography versus EUV lithography??????
3D X-ray lithography Multiple exposures with 3 different angles Photonic crystal
Electron beam lithography Since a long time one knows how to focus electrons beam spot < 10nm Very small wavelength: no diffraction limitation Direct writing: maskless sequential writing: small throughput resolution : depends on resist, one can reproduce the spot size i.e. 1nm
electron-resist interaction organic resist (PMMA) m m m m m m m m m m m m m m m m non soluble soluble Typical energy for breaking a bond: 10eV Typical energy of the beam : several 10keV (Problem of aberration at low energy)
Monte Carlo Simulation to study energy lost Forward scattering Substrat backscattering Spreading of the beam, lost of resolution Energy far from the impact of the beam, proximity effects
E(r) β a Double Gaussian model E( r) = aexp + exp r b r βa βb 2 2 β r r Tension kv 20 50 60 β a (µm) 0.08 0.04 - β r (µm) 2 9 13 β a forward scattering: Essentially depends on the resist and the voltage β r backscattering: Depends on the voltage and the substrat 120-43 Substrat Si
How to beat proximity effect Vary the dose depending on the pattern Use high energy: dilute proximity effect on a large area Use very small energy (STM) (but forward scattering) Use resist sensitive to high energy: inorganic resists Write on membranes
Proximity effects Dose depends on the pattern Intra proximity Dose depends on the surrounding of the pattern D=E(1+b) Real dose as exposed dose proximity effect
Software for proximity effect correction Commercial software exist (very expensive) Correction may needs negative doses at some points! It is very difficult to produce arrays of line with a very fine pitch
200kV e-beam lithography on PMMA Line <10nm Granular gold lift-off
Multilayer techniques resolution resist layer stoplayer absorber resist (low Z) Substrat (high Z)
Resolution of organic Resists
Inorganic resist sensitive to high energy Diffusion pump oil Polymerisation under the beam Size few nm (hard to remove!) Other inorganic resist: Al 2 O 3, NaCl, AlF 3, problems: very thin resist :no lift-off very high doses C/cm 2 i.e. 10 4 s/µm!
AlF 3 at 200kV
The e-beam writer (example of the LEICA 5000)
Schottky Emitter Tip <100> W Crystal ZrO Reservoir Polycrystalline tungsten heating filament Brightness >>LaB6 cathode Spot size<5nm at 500pA
Scanning Techniques for E-Beam Lithography Beam Scan Area Beam Scan Area Stage Movement Limits 1. Raster Scan The beam deflection system scans a fixed sized area whilst the beam is switched on and off to expose the local areas where shapes are required. 2. Vectorscan The blanked beam is deflected to the lower-left hand corner of a shape. The beam is unblanked and the required shape area then scanned. The beam is again blanked and deflected to the next required shape. 3. Stage Scan/ Static Beam The stage is moved in the path required to create the lithographic shapes while the beam remains undeflected Shaped beam for mask making machine
Vector Scan of Rectangle Shape Stop scan and blank beam Un-blank beam and start scan here Beam Step Size
Exposure Scan Strategy Main field double-lever scan coils deflect beam to start position of each shape. Final lens Main field
Exposure Scan Strategy The Trapezium Deflector scans the required lithography shape at the position within the Main Field set by the Mainfield deflector coils. Final lens Main field Trapezium shape maximum size (depends on EHT)
Trapezium Field The main reason for the Trapezium deflection system is speed. It is not possible to deflect the main beam with 25Mhz stepping frequency. Large current changes in inductive deflection coils require long settling times To achieve very fast deflection Use a coil with low self-inductance Limit the range of deflection currents Disadvantages: The deflection range is limited (12.8µm max but depends on EHT). Large shapes require fracturing into Trap deflection range sizes. Advantages: High speed deflection possible Exposure lost time for settling greatly reduced
Writing Strategy + Y Field Boundary Block Boundary Substrate on the Holder, on the Stage Trapezia - Positioned by main deflection - Written by Trapezia Scan 0,0 Fields/Blocks positioned by stage movement + X Field Size Shape positioning Resolution = 32768 Beam Step Size interval defines Trapezia size
Deflection Coils Basic Deflection System Main X Pattern Main Y Computer Generator Clock Trapezium Generator Trap X Trap Y Determine the dose Beam Blanking Beam Blanke r
Effects of deflection on the Beam Final Aperture Focal Plain Substrate Surface the pattern has to be divided into field
Laser Interferometer Optics Beam Bender 50% Beam Splitter Y Axis Remote Interferometer Laser Optics Box Y Axis Receiver Y Axis Mirror Stage X Axis Laser X Axis Mirror Stage Stage Y Axis Main Chamber Airlock Beam Bender X Axis Receiver X Axis Remote Interferometer The Laser emits a second beam for each axis which is polarized at 90 0 to the first. This beam travels through a different path as shown. It is reflected back to the Receiver by the Remote Interferometer optics and does not see the Stage. This beam measures any changes of path length between the Laser and the Remote Interferometer units. The measurements of the two beams are combined and the resultant signal output provides an accurate measurement of the position of the stage relative to the remote interferometer units. Hence changes of room temperature affecting the path length in the Laser Optics Box do not affect the accuracy of the measurement of the Stage position. Accuracy about 2nm
Elements of Beam Error Feedback (Pull-in) R Required Stage Position M Mechanical Stage Position R- M BEF Stage Position Values DAC Laser Interferometer Calibration Scale and Rotation Stage Mirrors Amplifier (R - M) E-Beam Deflection Coils Stage Mechanical Stage position Required Destination
e-beam lithography: Highest resolution Low process - not for industrial purpose (for all processes) Intermediate cost : 150k for SEM based equipment 3M for e-beam writer
Ion beam lithography Revival of ions beam spot size < 10nm Ions are rapidely absorbed no proximity effect Small doses Tridimensionnal structures Direct writing (without resist) through etching or implantation.
Ion trajectories
30kV Gallium ions Holes in a Si 3 N 4 membrane 10 nm LPN Marcoussis
Ion beam lithography on AlF3 resist 30kV Ga ion
3D lithography on organo-metallic gold composite Résist:Au 55 (PPh3) 12 Cl 6 Dimensions : 30 nm wide, 20 nm height : 1.5 µm long. (Ga ions, energy 30 kev, initial thickness 50nm)
Local FIB induced mixing - Thin magnetic films patterning FIB probe Pattern Pt (3,4 nm) Co (1,4 nm) Pt (4,5 nm) Transparent alumina substrate Magneto-optical image of magnetic domains defined between irradiated lines (Ga + ions, 30 kev, 5 10 15 ions/cm 2 ). Arrays of stable magnetic dots 1500 nm, 750 nm, 300 nm, 50 nm
Tridimensional etching
Near field lithography
Near field lithography through local electrochemistry example of gold a) Surface water condensation b) Monolayer of oxydize gold H 2 O Gold surface oxygen atoms gold atoms c) Exchange process d) Dissolution of gold atoms
examples
Near field scheme Electrical pulse Mechanical pressure Below threshold threshold Observation/alignment
Local CVD deposition PF 3 PF 3 Rh Cl Cl Rh PF 3 PF 3 depassivation deposit low pressure one pulse one atome 100nm GPEC Marseille
Example of useful structures Anodization of GaAs Anodization of Nb ETH Zürich CRTBT
Use carbon nanotube to improve the resolution Pb vibrations needs short tube 0.2µm LEPES Grenoble
Slow process parallel set-up
Thermal lithography Milliped project IBM Zürich
Dip pen lithography Application to DNA Chip resolution =40nm Northwestern Univ
Nano-imprint 1.temp +pressure 50Bars resist mold substrate 2. cooling 3. Remove mold (tricky!) 4. Etch of residual resist Slow process, Need mask at 1/1 scale i.e. e-beam lithography Resolution demonstrated down to 10nm. Very chip!
examples
UV assisted imprint Quartz mold substrate UV hardening of the resist Much faster, still problem for alignment, commercial systems now
Nano-stamp Use of molecular adhesion Example : thiol group on gold PDMS ink etch thiols Gold Si P = 400 nm
Conclusion on lithography techniques Technique Resolution Use Remarks contact 0.25µm Labs and R&D Economical Optical lithography proximity 2µm Labs and R&D projection 80nm Industrial Economical but weak resolution Expensive but with constant progress EUV <50nm Industrial May be the next tehnique for 2005 Electron lithography 1nm Labs andr&d Fabrication of optical masks Technique without mask best resolution Lithographie ionique 8nm Labs and R&D Better for etchig than lithography (diagnostic) Near field lithography Atom 10nm Labs Economical, very slow specific Nanoimprint 10nm Labs and industry? Economical, fast Alignment problems mask 1 :1
Transfert techniques Wet etching Ion Beam Etching Reactive Ion Etching Reactive Ion Beam Etching Dense plasma
isotrope wet etching Simple Fast Do not respect the design rule Wet etching You may think to use under etching to reduce thee size. Difficult to control because of surface state: strong etching (not sensitive to surface state) too fast Weak etching slow but too sensitive to surface state
Anisotropic wet etching Use anisotropic etch rate with crystal face Still some under-etch Use to produce nice features over-growth in V-groves Can be mixted with stop layer
Ion Beam Etching IBE gas Use the impact of impining ions. Purely physical Sputtering rate T accelerated ions T ZU E U binding energy of material Z atomic number of mateerial E ion energy x coeff (angle)
Quite slow No selectivity Re-deposition Trenching damage
Reactive ion etching: RIE rf plasma C Autopolarisation few100v Chemically active ions
passivation gas Anisotropy achievement
Avantages of RIE Fast proceess Selectivity Anisotropy No redeposition Use of passivation layer problems of RIE Sensitive to pollution Energy and pressure are linked
Reactive Ion Beam Ething:RIBE Same as IBE but with chemically active ions Allows to separate the physical/chemical action Impressive aspect ratio
Examples RIE 1,94µmby 6,25µm AlAs/GaAs miropillar 7.5 µm Depth limited to 1.2mm For 0.4mm diameter holes
Example RIBE
Electron Cyclotron Resonance and Inductive Coupled Plasma High density plasma (fast) with low energy (damage) Independant control of energy/density
Top down and bottom up? Both techniques tend to the same dimension Future of nanotechnology will be certainly a mixing of these techniques Addressing of individual macromolecules Structuration of substrat
Carbone nanotube and e-beam lithography 19000 18000 17000 16000 15000 T=30 mk 14000 0 20000 40000 60000 80000 Magnetic field (Gauss) LPN-Marcoussis
CVD growth of Carbone Nanotube on structured catalyst LEPES Grenoble
Cluster deposition on structurated substrat FIB structurated substart and gold cluster deposition (coll. DMP Lyon - LPN)